1. Field of the Invention
The invention relates to a nickel-diamond coated saw wire with improved anchoring of the diamond particles in the metallic binder phase.
2. The Prior Art
In DE A 19839091 (corresponds to the U.S. application bearing Ser. No. 09/332722), a nickel-diamond coated saw wire, a process for its production and its use are described. A wire of this type can be used to saw hard, brittle material, such as silicon. The saw wire is produced by introducing a wire, after a chemical pretreatment, into a chemical nickel bath which contains diamond particles, preferably of a mean diameter of from 5 to 30 xcexcm. The diamond particles which are deposited on the surface of the wire are initially fixed by weak chemical and/or physical bonding forces. During the coating process, the adhering diamond particles are firmly enclosed by the growing nickel layer and are thus securely anchored. The thickness of the metallic binder phase is preferably 5-20 xcexcm. The properties of the metallic binder phase are considerably improved by subsequent thermal hardening. For example, a reduction in the residual tensile stresses in the metallic binder phase is observed, the adhesive strength is increased and the hardness of the layer rises considerably. When sawing silicon, a wire of this type provides very good sawing results, as can be seen from the abovementioned application.
On account of the properties described, the nickel-diamond coated saw wire also appears to be suitable for sawing hard ceramic materials, such as silicon carbide. Although tests have shown that in principle a saw wire of this type can be used to cut hard ceramic materials, the high hardness of the hard ceramic materials means that the sawing rates which can be achieved and also the service life of the coated saw wire are insufficient for industrial use.
It is an object of the invention to provide a diamond-coated saw wire which has a high resistance to wear when sawing hard materials, such as for example hard ceramic materials.
The object is achieved by a saw wire, comprising a steel wire, an intermediate layer and a metallic binder phase in which diamond grains, with a mean diameter of 10 to 50 xcexcm are embedded, the intermediate layer being undamaged and free of diamond grains and serving both to prevent hydrogen embrittlement of the wire and to ensure sufficient adhesive strength of the metallic binder phase, characterized in that the metallic binder phase has a hardness of between 600 and 1100 HV 0.1 and comprises an inner layer and an outer layer, which are arranged concentrically around the steel wire which has been provided with the intermediate layer, the inner layer having a thickness of approximately 10 to 25% of the mean diamond grain diameter and the outer layer having a thickness which is such that the total thickness of the metallic binder phase is 45-55% of the mean diamond grain diameter, and the diamond grains having a mean spacing between them of no more than five times their mean diameter and further fine particles with a mean diameter of from 1 to 6 xcexcm being situated between the diamond grains with a mean diameter of from 10 to 50 xcexcm.
The diamond grains which are fixed to the saw wire are therefore anchored in the metallic binder phase approximately as far as their equator.
The diamond grains which are fixed to the wire preferably have a mean diameter of from 25 to 45 xcexcm. Said diamond grains are preferably positioned on the wire in a quantity which is such that their mean spacing amounts to no more than 0.5 to 3 times their mean diameter.
Further fine particles are situated between the diamond grains of mean diameter of from 10 to 50 xcexcm. These fine particles are preferably hard-material particles. They have a mean diameter of from 1 to 6 xcexcm, preferably 2 to 4 xcexcm. These fine particles are preferably only present in the upper of the two layers of the metallic binder phase.
These relatively small hard-material particles are preferably likewise diamond grains. The mean diameter of these fine diamond grains is not taken into account when establishing the layer thickness of the binder phase (which according to the invention is 45-55% of the mean diamond grain diameter). Only the mean diameter of the coarser diamond grains is used to determine the layer thickness of the binder phase.
Including the coating of abrasive grains, the saw wire according to the invention preferably has a diameter of up to at most 0.35 mm.
The steel wire used is preferably stainless spring steel wire made from chromium-nickel steel. By way of example, material types 1.4310, 1.4401, 1.4539, 1.4568 and 1.4571 (as designated by DIN 17224) are suitable.
The steel wire preferably has a diameter of from 0.15 to 0.30 mm.
The intermediate layer preferably consists of a metal, a metal alloy or a combination of two metals or one metal and one metal alloy.
It is preferably a metal, a metal alloy or a combination of two metals or of one metal and one metal alloy which can be coated by electrodeposition with good adhesive strength and, at the same time, acts as a hydrogen barrier.
Examples of suitable metals are copper and nickel and an example of a suitable metal alloy is brass.
The intermediate layer preferably has a thickness of from 1 to 10 xcexcm.
The metallic binder phase preferably consists of nickel or nickel alloys. It particularly preferably consists of nickel which has been deposited without external current (chemical nickel).
The metallic binder phase preferably has a hardness HV 0.1 of from 800 to 1100, particularly preferably from 1000 to 1100.
The thickness of the intermediate layer around the wire preferably varies by no more than 5%.
The thickness of the metallic binder phase around the wire preferably varies by no more than 5%, particularly preferably by no more than 2.5%.
In a particularly preferred embodiment, the layer thickness of the metallic binder phase is from 12 to 15 xcexcm and the thickness of the metallic binder phase around the wire varies by at most 0.4 xcexcm.
The saw wire according to the invention can be produced as follows:
As is already known from DE A 19839091, a wire which has been provided with an intermediate layer undergoes a chemical pretreatment which is adapted to the base material. This pretreatment is known in the prior art and usually comprises known degreasing, pickling and activation treatments. There then follows the coating in a so-called chemical nickel-diamond bath (bath for nickel-diamond deposition without external current). Baths of this type are likewise known in the prior art. As is also known from the prior art, it may be useful to initiate the commencement of the metallization by means of a short surge of current. Uniform diamond embedding over the entire circumference of the wire is achieved by means of a suitable movement of wire and electrolyte. In the process according to the invention, it is essential that the coating takes place in two stages, diamond particles with a mean diameter of 10-50 xcexcm being deposited on the wire in the first stage and fine particles with a mean diameter of 1 to 6 xcexcm being deposited on the wire in the second stage.
By means of the process according to the invention, it is possible within one hour to produce more coated wire by a factor of 103 than by means of electrodeposition in continuous throughput installations with a comparable installation size.
The coating is preferably followed by a heat treatment for at least one hour.
In the process according to the invention, the two-stage coating of the wire following the chemical pretreatment preferably takes place as follows: In the first coating stage, the wire is introduced into a chemical nickel bath which contains diamond particles with a mean diameter of from 10-50 xcexcm, preferably 25-45 xcexcm. The coating process is controlled in such a way that, in the ideal situation, a single layer of diamond grains is deposited on the surface of the wire and these grains are fixed by the growing layer of nickel during the process. The first coating stage has finished as soon as the nickel layer has reached a thickness of 10 to 25% of the mean diamond grain diameter.
In the second coating stage, a further chemical nickel layer is deposited, which contains particles which are significantly finer than the first chemical nickel layer.
The thickness of the chemical nickel layer with fine particles embedded therein which is deposited in this second process stage is selected in such a way that the overall layer thickness including the layer which was deposited in the first coating step preferably amounts to 45-55% of the mean diameter of the diamond grains which were used in the first process step. Therefore, given the approximately equiaxial form of the diamond grain, good anchoring up to approximately the equator of the diamond grains is achieved in the metallic binder phase.
In this bath, the fine particles have a mean grain diameter of from 1 to 6 xcexcm, preferably from 2 to 4 xcexcm.
They are preferably hard-material particles, among which in turn diamond particles are preferred.
Therefore, in this second coating stage the coarse diamond grains are anchored using a further chemical nickel dispersion layer.
In principle, it is also possible for the entire metallic binder phase which is required in order to fix the coarse diamond grains on the wire to be formed as a dispersion layer with embedded coarse diamonds and fine hard-material particles. The coating process for producing a wire of this type is in this case carried out as described in DE 19839091, both coarse and fine grains being added to the coating bath simultaneously. Although a production process of this type is simpler to carry out than the process according to the invention, the wire obtained in this way has the serious drawback that it loses its cutting properties prematurely, in particular when sawing hard materials, for the following reason: The coarse diamond grains which are incorporated toward the end of the residence time of the wire in the coating bath are only slightly fixed on account of their weak anchoring in the binder phase. They are quickly torn out during sawing. This immediately leads to the cutting ability of the wire being lost.
As has already been described in DE-A-19839091, after the coating the saw wire is usually subjected to a heat treatment, preferably at 350xc2x0 C., in order to reduce residual tensile stresses in the layer and to increase the layer hardness and therefore the resistance of the metallic binder phase to wear.
Since, during the anchoring of the coarse diamond particles in a further dispersion layer, there is already a layer which is under residual compressive stresses, it is preferable to carry out a heat treatment exclusively with a view to increasing the adhesive strength of the layer. To do this, a heat treatment temperature of from 150-200xc2x0 C. is sufficient.
This low heat treatment temperature has the advantage that even suitable unalloyed high-strength wires can be coated using the process according to the invention and can be thermally treated at 150-200xc2x0 C. without a critical drop in the tensile strength and elasticity.
Electrodeposition processes are unsuitable for production of the wire according to the invention, since they are unable to achieve the hardness according to the invention of the metallic binder phase and the low fluctuation in thickness of the metallic binder phase which is preferably present.